Fact or Fiction? Gas Myths Debunked

Roll down your windows and turn off the A/C. Use premium gas. Fill up your tank early in the day.

As cost-conscious drivers gather around the ever-gouging gas pump, these are just a few of the oft-repeated tenets of fuel-saving wisdom. But according to experts, many of these so-called tips are mere myths. Gearheads from the American Automobile Association, Consumer Reports and "Mythbusters" helped us sort fact from fiction.

Your Friends Say: Air Conditioning Drains Your Gas TankExperts Say: It's So Insignificant, You Might as Well Be Comfortable

One of the most commonly-heard myths touted by consumers and Manhattan cab drivers alike is that running the A/C drains your gas tank. Not true, according to Michael Calkins, the manager of approved auto repair at AAA — at least not on the highway.

According to Calkins, on the highway, both air conditioning and open windows affect fuel economy in similar ways: only about half a mile per gallon.

New York cab drivers aren't completely wrong. In stop-and-go traffic, open windows don't have nearly as much drag, so open windows may rob less fuel than the air conditioner.

"A/C uses slightly more gas, but the savings are not really significant," Caulkins said. "But when you consider the fatigue factor, it's better to use the air conditioning though it may cost you half a mile a gallon. — If ultimate fuel efficiency is your only goal, then driving with your windows down will save you a tiny bit of fuel around town certainly, but not on the highway."

Your Friends Say: Buy Gas Early in the Day Experts Say: Buy Gas When You Need It

Another common gas pump mystery surrounds the time of day to buy gas. The thinking goes something like this: As it gets hot, gas expands in the tank, so if you buy early in the day, you'll get more bang for your buck.

But tailoring your gas-buying habits to this idea isn't particularly useful, according to David Champion, director of automobile testing at Consumer Reports.

"The hotter the day … the more gas will evaporate as you put it into the car. When your car is nice and cool, there's less evaporation as you put it into the car," Champion said. "It doesn't make much of a difference to the individual consumer. … It's 1 percent [of gas] a year that you may use."

According to Calkins, no matter what time of day you buy it, gas is generally always going to be the same temperature.

"[Most] gas is stored in underground tanks," he said. "It doesn't change in temperature at all."

Your Friends Say: Premium Gas Gets Better Gas MileageExperts Say: If Your Car Can Take a Lower Grade, Go For It and Save

While AAA's Calkins generally advises people to follow their owner's manual, this is one area where in regular driving conditions he says that drivers can deviate.

"The reality is manufacturers' fuel recommendations are based on the performance potential of the engine," he said. "On modern cars, even when it says to use premium fuel, mid-grade or regular fuel will work. It will reduce your performance, but if you're not going to be towing a trailer or racing, you don't necessarily need 100 percent of the power."

Calkins says that as long as you don't hear a knocking sound, you have no problem.

"If you do hear that, you should immediately go back to a higher grade," he said.

No matter what you see in the back ads of "Popular Mechanics," don't buy any additive or gadget that claims to increase gas mileage, experts say.

"I can sum up all of these devices in one sentence: None of them work," Calkins said. "The EPA has a complete test procedure for anything that claims it increases fuel economy. To my knowledge, not one single manufacturer has ever successfully completed that protocol and been able to show scientifically [it works]. As much as we all want to believe in the tooth fairy and the Easter bunny, you can't buy them on the shelf at the auto parts store."

Champion concurred, saying the magazine has tested many of these devices and never found one that worked.

"Mythbusters" co-host Adam Savage has made a living debunking gas-saving myths on his popular Discovery channel show. The only thing he's found that really saves gas? Relaxing. In an experiment that will air in an upcoming show, the "Mythbusters" team found that people who drive while they're angry use 50 percent more gas than people who are relaxed.

"The difference was absolutely astonishing," Savage said. "You just use a lot more gas [when you're angry]. That has more of an effect on your fuel economy than any other single thing."

There's no magical trick to saving gas, the experts told ABCNews.com; it's all common sense.

"Drive as if you have a cup of coffee on your dashboard," Champion said. Smooth acceleration and deceleration improve fuel economy as does driving while your car is "warm."

"Put all your trips together," Calkins said. "Cars use a lot of fuel when they're cold."